We start with the standard DSM definition of delusions, and explain that clinical delusions are characterised by surface features of two kinds, epistemic (fixity, implausibility) and psychological (negative impact on functioning). Then we ask whether we can decide whether a type of belief is delusional by using those criteria. We consider three cases of belief that match at least some of the criteria: the belief that some thoughts have been inserted in one's mind by a third party; the belief that one has been abducted by aliens; and the belief that one is better than average at just about everything.

Discussing the similarities and differences among such beliefs -- Are they based on evidence? Are they common or rare? Do they bring any benefit? Do they help explain anomalous experience? -- is relevant to two projects that have been featured on this blog before: the Costs and Benefits of Optimism project and the PERFECT project. In the end, our goal is to show that neither the epistemic nor psychological features mentioned in the definition of delusions can help us sort delusional beliefs from non-delusional ones. Some delusions do not meet the criteria and some beliefs that we would not class as delusions do.

Rachel Gunn (who successfully completed her PhD at the University of Birmingham, working on delusions) examines the phenomenon of thought insertion by using very interesting first-person reports by people who used internet forums to share their unusual experiences. She observes that the experience affects people in different ways. For some but not for all the thought is felt like an unwelcome intrusion. For some but not for all the content of the thought also generates a compulsion to think or do certain things. Thought insertion is regarded as a symptom of schizophrenia, but whether it involves a pathological experience, whether it is a delusional belief, or whether it is harmful cannot be established without reflecting on the specifics of the individual case.

Here is one of the first-person reports Rachel analyses:

i truly do have unwanted thoughts that are forced into my head from somewhere... I mean I will have a thought saying my grandmother is a bitch. I would never ever think of my grandmother as a bitch. She is one of the greatest women I know and I adore her. So how is that a delusion? It is an intrusive thought! I sure didnt imagine it!.... i really do not think my grandmother is a bitch. i think these thoughts are evil and came from an evil being. Some thoughts however that pop into my head all of a sudden are my own thoughts and i can recognize that even though they are unwanted, but some are just plain ridiculous and mean and i know must be from an outside force.

Star-28, ‘Forum’, Mental Health Forum (2010)

Ema Sullivan-Bissett (lecturer in Philosophy at the University of Birmingham) reviews the fascinating literature on alien abduction beliefs, discussing some hypotheses about the experiences and background beliefs that contribute to people believing in abduction. Ema notices that although alien abduction beliefs are not technically delusions due to the fact that they are widespread in some subcultures, and delusions are supposed to be idiosyncratic, alien abduction beliefs share many epistemic features (implausibility, resistance to revision, etc.) and psychological effects (distress) with delusions.

Here is one of the reports of alien abduction belief from the literature:

While lying helplessly on your back and unable to cry out for help, you become aware of sinister figures lurking in your bedroom. As they move closer to your bed, your heart begins to pound violently and you feel as if you are suffocating. You hear buzzing sounds, and feel electrical sensations shooting throughout your body. Within moments, the visions vanish and you can move again. Terrified, you wonder what has just happened. (McNally and Clancy, ‘Sleep Paralysis, Sexual Abuse, and Space Alien Abduction’, p. 114)

My job in the paper is to argue that there are strong epistemic similarities and some similarities in psychological effects between delusions and positive illusions, despite the latter being extremely common in the non-clinical population and regarded as generally adaptive. Positive illusions have been branded as mild distortions of reality and as less fixed than delusions, but they are not well supported by evidence and they are implausible too. Examples are the belief that one is better than average in a variety of domains, or that one will not experience negative events (such as divorce or serious illness) in the future. In some contexts such illusions enhance wellbeing and mental health, but in other contexts can lead people to take unnecessary risks and experience disappointment and frustration.

The conclusion is that it is more challenging than expected to tell delusional beliefs apart from other beliefs, because there is no one 'mark of madness'.